Grazia Monni - Academia.edu (original) (raw)

Papers by Grazia Monni

Fusion Engineering and Design, 2015

Different configurations of water-cooled Cu collector for gyrotrons are investigated using the St... more Different configurations of water-cooled Cu collector for gyrotrons are investigated using the Star-CCM + CFD code, aimed at optimizing its thermal-hydraulic (TH) performance. Although the current collectors show a good performance, the collector can be subjected to transient heat loads, due to the spent electron beam, of up to several tens of MW/m 2 , and there is an interest to increase the gyrotron output power in the future. Furthermore, an optimized cooling will lead to improved reliability and lifetime of the collector. Starting from a hypervapotron (HV)-like collector, characterized by 100+ deep rectangular cavities with aspect ratio (AR) = 3, we present in the first part of the paper a single-cavity steady-state parametric analysis of the effect of AR on the heat exhaust capabilities. The investigation is then extended to other collector designs, including circumferential ribs and dimples, in order to assess the options for further improvements of the TH performance. The peak Cu temperature is computed by the code and its minimization is the target of the present optimization exercise. A self-consistent estimate of the heat transfer coefficient between collector and coolant is also obtained, which could be useful for fatigue and lifetime assessments. In the second part of the paper the most promising collector geometries identified in the first part are analyzed in the case of a transient heat load (vertical sweeping), first at the level of a single spatial period of the collector structure, then at the full-collector level. The results of the TH transient analysis are compared with both the results of the first part and with the transient purely thermal analysis of the full collector, showing for all geometries considered in this study a room for cooling efficiency improvement with respect to the HV-like design with AR = 3, at least in the operating conditions considered for this study (V ∼ 4 m/s, almost 100 • C sub-cooling).

Questo rapporto presenta l’attivita teorica e sperimentale svolta presso il Dipartimento Energia ... more Questo rapporto presenta l’attivita teorica e sperimentale svolta presso il Dipartimento Energia del Politecnico di Torino nello sviluppo della modellazione di uno spool piece (SP) per la stima dei parametri caratteristici di un deflusso bifase sulla base delle caratteristiche dello SP, della procedura di prova e dei risultati ottenuti nel corso delle campagne sperimentali Il dispositivo preso in considerazione e costituito da un tubo di Venturi e dalla sonda ad impedenza sviluppata dalla SIET per la stima della frazione di vuoto. I deflussi di interesse sono quelli ad elevata frazione di vuoto con flow patterns che coinvolgono il moto anulare, il moto anulare – disperso, il moto stratificato. Per integrare la matrice sperimentale ottenuta dal POLITO nella precedente annualita con la sonda in configurazione verticale, e stata condotta una campagna sperimentale con la stessa montata in configurazione orizzontale composta di 30 prove sperimentali in regime di “wet gas” (grado di vuoto...

The experimental analysis and the qualification of an Electrical Capacitance Probe (ECP), for the... more The experimental analysis and the qualification of an Electrical Capacitance Probe (ECP), for the void fraction measurement, is presented. The ECP, developed at the SIET, is used to investigate a vertical air/water flow, characterized by high void fraction. Mass flow rates have been analyzed between 0.094 and 0.15 kg/s for air and between 0.002 and 0.021 kg/s for water, corresponding to void fraction up to 90% and to annular and wavy-annular flow patterns. The ECP signals are used to obtain the geometrical shape functions (signals as a function of electrode distances) in single and two phase flows. The dependence of the signal by the void fraction is derived and other flow characteristics as the liquid film thickness and the phase velocities are evaluated by means of rather simple models. The experimental analysis allows to characterize the ECP, showing the advantages and the drawbacks of this technique for the two-phase flow characterization at high void fraction, and it provides a...

Questo rapporto presenta l’attivita teorica e sperimentale svolta presso il Dipartimento Energia ... more Questo rapporto presenta l’attivita teorica e sperimentale svolta presso il Dipartimento Energia del Politecnico di Torino per la caratterizzazione della strumentazione per la misura della portata in un deflusso bifase. Tale strumentazione, che consiste in uno ”Spool Piece” formato da un Venturi per la misura delle cadute di pressione ed una sonda ECP (Electrical Capacitance Probe) per la misura del grado di vuoto, e stata sviluppata per la misura della portate di fuga in simulatori integrati come lo SPES3. Studi condotti dal DEP nei PAR precedenti hanno mostrato la possibilita di utilizzare la misura delle cadute di pressione misurate dal Venturi per ricavare le portate delle due fasi una volta noto il modello di flusso bifase. Tale informazione puo essere ricavate dal segnale della sonda ECP ma occorre sviluppare una metodologia che permetta di interpretarlo correttamente. Nel corso del PAR2013 il DEP aveva testato lo “Spool Piece” in configurazione orizzontale, con un deflusso bi...

Nell’ambito dell’attivita LP2 “Reattori evolutivi INTD” del PAR2011, con riferimento ai reattori ... more Nell’ambito dell’attivita LP2 “Reattori evolutivi INTD” del PAR2011, con riferimento ai reattori di piccola e media taglia, il Dipartimento Energia del Politecnico di Torino, ha svolto una attivita di ricerca teorico-sperimentale con lo scopo di sviluppare e caratterizzare la strumentazione per la misura della portata in un deflusso bifase, da utilizzare nell’impianto SPES3. L’impianto SPES3 (Simulatore Pressurizzato per Esperienze di Sicurezza) e un simulatore integrato di reattori di piccola taglia modulari (SMR), capace di verificare la risposta del reattore a fronte di transitori incidentali e fornire dati sperimentali per la validazione di codici di calcolo per le analisi di sicurezza. Il circuito SPES-3, opera a elevate pressioni e temperature, e con elevate velocita dei fluidi. A causa delle condizioni di funzionamento, nella simulazione della rottura di una tubazione, si instaura un deflusso bifase nei condotti di collegamento tra il punto di rottura ed il componente di racc...

Nel presente documento e descritta la caratterizzazione di uno Spool Piece costituito da un ventu... more Nel presente documento e descritta la caratterizzazione di uno Spool Piece costituito da un venturi classico e da un Wire Mesh Sensor, installati in un condotto orizzontale interessato dal deflusso di una miscela aria-acqua. Questo Spool Piece e stato installato e testato in un circuito realizzato presso il laboratorio del Dipartimento Energia del Politecnico di Torino, che consente di realizzare un deflusso bifase aria-acqua in un ampio range di portate, permettendo lo studio di deflussi caratterizzati da differenti condizioni fluidodinamiche. Lo studio ha consentito di approfondire i lavori effettuati dal Politecnico di Torino nell'ambito dell'attivita LP2 "Reattori evolutivi lNTD" del PAR2008-09, in particolare nell'ambito della scelta degli strumenti da installare nelle linee di rottura di SPES3 con l'obiettivo di misurare la portata massica bifase delle due fasi che transita nelle linee durante i transitori incidentali. L'analisi dei segnali elettr...

Journal of Physics: Conference Series, 2015

The paper presents the results obtained with a spool piece (SP) made of a Venturi flow meter (VMF... more The paper presents the results obtained with a spool piece (SP) made of a Venturi flow meter (VMF) and an Electrical Capacitance Probe (ECP) in stratified two-phase flow. The objective is to determine the relationship between the test measurements and the physical characteristics of the flow such as superficial velocities, density and void fraction. The outputs of the ECP are electrical signals proportional to the void fraction between the electrodes; the parameters measured by the VFM are the total and the irreversible pressure losses of the twophase mixture. The fluids are air and demineralized water at ambient conditions. The flow rates are in the range of 0,065-0,099 kg/s for air and 0-0,039 kg/s (0-140 l/h) for water. The flow patterns recognized during the experiments are stratified, dispersed and annular flow. The presence of the VFM plays an important role on the alteration of the flow pattern due to wall flow detachment phenomena. The signals of differential pressure of the VFM in horizontal configuration are strongly dependent on the superficial velocities and on the flow pattern because of a lower symmetry of the flow with respect to the vertical configuration.

Transactions of the American Nuclear Society

In this study, the phase distribution of an air-water bubbly flow in a horizontal test section wi... more In this study, the phase distribution of an air-water bubbly flow in a horizontal test section with an inner diameter of 19.5 mm has been experimentally investigated by means of a Wire Mesh Sensor (WMS). The experimental test section is equipped with two quick closing valves (QCV) that allow the measurement of the volumetric void fraction in a length of 1300 mm. Two phase flow experiments have been performed at a constant temperature of 20 ± 2°C. In the present tests the superficial velocity ranges from 0.23 to 0.5 m/s for air and from 1.4 to 2.62 m/s for water.

Progress in Nuclear Energy, 2014

An experimental investigation of a vertical upward annular two-phase flow across a Venturi flow m... more An experimental investigation of a vertical upward annular two-phase flow across a Venturi flow meter has been performed for the measurement of two-phase flow parameters with reference to the experimental simulation of nuclear accidents, as LOCA, characterized by very high void fraction. The pressure drops between the inlet and throat section and between inlet and outlet (irreversible pressure losses) have been measured and analyzed. The Venturi flow meter is characterized by an inlet diameter of 80 mm and a throat diameter of 40 mm (b ¼ 0.5), with equal convergent and divergent angles (q ¼ 21). The instrument has been tested in a test section having an internal diameter (D i) of 80 mm and a total length (L) of about 4 m, with airewater two-phase flow at ambient pressure. The air superficial velocity ranged between 14 and 18 m/s while the water superficial velocity ranged between 0.0008 and 0.005 m/ s, so that the flow pattern was annular and the corresponding flow quality ranged between 0.78 and 0.96. The dependence of the pressure drops on the phases velocities has been analyzed and modeled as a function of the superficial velocities of the phases, and the mass flow rate of the two phases has been derived from the present test data when the annular flow pattern does not change in a significant way. The flow quality can be evaluated with an accuracy of 5% and the mass flow rate of air and water can be estimated with an accuracy better than 2% and 30% respectively.

Journal of Physics: Conference Series, 2014

The paper presents the research work performed at the Energy Department of the Politecnico di Tor... more The paper presents the research work performed at the Energy Department of the Politecnico di Torino, concerning the development of two-phase flow instrumentation and of models, based on the analysis of experimental data, that are able to interpret the measurement signals. The study has been performed with particular reference to the design of power plants, such as nuclear water reactors, where the two-phase flow thermal fluid dynamics must be accurately modeled and predicted. In two-phase flow typically a set of different measurement instruments (Spool Piece – SP) must be installed in order to evaluate the mass flow rate of the phases in a large range of flow conditions (flow patterns, pressures and temperatures); moreover, an interpretative model of the SP need to be developed and experimentally verified. The investigated meters are: Turbine, Venturi, Impedance Probes, Concave sensors, Wire mesh sensor, Electrical Capacitance Probe. Different instrument combinations have been tested, and the performance of each one has been analyzed.

Computational Methods in Multiphase Flow VII, 2013

A Wire Mesh Sensor (WMS), based on the measurement of the local instantaneous conductivity of the... more A Wire Mesh Sensor (WMS), based on the measurement of the local instantaneous conductivity of the two-phase mixture, has been adopted to characterize the air-water flow in a test section consisting of a horizontal Plexiglas (transparent) pipe of internal diameter 19.5 mm and total length of about 6 m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.145 to 31.94 m/s for air and from 0.019 to 2.62 m/s for water; the pressure ranges from atmospheric pressure to 3.7 bar. The observed flow patterns are stratified-bubble-slug/plug-annular. The void fraction profiles are derived from the sensor signals and their evolution in time and space is analyzed and discussed; the dependence of the signals from the measured fluid dynamic quantities is discussed too.

Journal of Physics: Conference Series, 2014

This works presents the characterization of a Spool Piece (SP), made up of a Classical Venturi an... more This works presents the characterization of a Spool Piece (SP), made up of a Classical Venturi and a Wire Mesh Sensor (WMS), that are installed in a horizontal test section, in which an air-water mixture flows. The test section consists of a horizontal Plexiglas pipe of internal diameter 19.5 mm and total length of about 7 m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.14 to 32 m/s for air and from 0.019 to 2.62 m/s for water; the pressure ranges from atmospheric pressure to 4 bar depending on the experimental conditions. The observed flow patterns are stratified-bubbly-slug/plug-annular. The instruments response is analyzed and discussed. From the signal analysis the mass flow rate of each phase is obtained. The developed model allows the evaluation of the mass flow rate with an accuracy higher than 20% in the 84% of the cases and with an accuracy higher than 10% in the 73.3% of the cases. Finally the introduction of the estimated parameters in the SP model is considered and discussed.

Journal of Physics: Conference Series, 2012

ABSTRACT A Wire Mesh Sensor, based on the measurement of the local instantaneous conductivity of ... more ABSTRACT A Wire Mesh Sensor, based on the measurement of the local instantaneous conductivity of the two-phase mixture, has been used to characterize the fluid dynamics of the gas-liquid interface in a horizontal pipe flow. Experiments with a pipe of a nominal diameter of 19.5 mm and total length of 6 m, have been performed with air/water mixtures, at ambient conditions. The flow quality ranges from 0.00016 to 0.22 and the superficial velocities range from 0.1 to 10.5 m/s for air and from 0.02 to 1.7 m/s for water; the flow pattern is stratified, slug/plug and annular. A sensor (WMS200) with an inner diameter of 19.5 mm and a measuring matrix of 16×16 points equally distributed over the cross-section has been chosen for the measurements. From the analysis of the Wire Mesh Sensor digital signals the average and the local void fraction are evaluated and the flow patterns are identified with reference to space, time and flow rate boundary conditions.

Experimental Thermal and Fluid Science, 2014

In the present work a Wire Mesh Sensor (WMS) has been adopted to characterize the air-water two-p... more In the present work a Wire Mesh Sensor (WMS) has been adopted to characterize the air-water two-phase flow in a test section consisting of a horizontal Plexiglas pipe of internal diameter 19.5 mm and total length of about 6 m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.145 to 31.94 m/s for air and from 0.019 to 2.62 m/s for water. The observed flow patterns are stratified-bubble-slug/plugannular. The WMS consists of two planes of parallel wire grids (16x16) that are placed across the channel at 1.5 mm and span over the measuring cross section. The wires of both planes cross under an angle of 90°, with a diameter D wire of 70 μm and a pitch equal to 1.3 mm. The void fraction profiles are derived from the sensor data and their evolution in time and space is analyzed and discussed. The dependence of the signals on the measured fluid dynamic quantities is discussed too. The main task is to predict which flow pattern will exist under any set of operating conditions as well as to predict the value of characteristic flow parameters.

Fusion Engineering and Design, 2015

Abstract The present gyrotron designs for EC plasma heating in nuclear fusion reactors require th... more Abstract The present gyrotron designs for EC plasma heating in nuclear fusion reactors require the safe exhaust of a power comparable to that injected into the plasma, in order to keep the maximum temperature below the acceptable value of 300 °C. In this paper, the commercial computational fluid dynamics (CFD) software STAR-CCM+® is used to analyze the thermal performance of the annular copper collector of a 1 MW ITER gyrotron, equipped with a hypervapotron structure made of annular fins with rectangular cavities of aspect ratio (depth/width) = 3, cooled by highly subcooled (90–100 °C) pressurized water flowing at ∼4 m/s. It is assumed that the simple vertical sweeping strategy is used to reduce the very high peak heat flux on the collector (up to 30 MW/m2 transient, 5 MW/m2 time average), due to the spent electron beam. The 2D steady-state conjugate heat transfer problem is solved assuming azimuthal symmetry and accounting for 2-phase flow. The single-cavity flow and heat transfer problem is considered first, to optimize the mesh and the selection of the turbulence model. For the operating conditions considered in this paper, the full collector (100+ cavities) solution shows that boiling occurs only in a limited number of cavities close to the peaks of the heat flux, with the vapor remaining trapped in the bottom of the cavities, i.e. no full hypervapotron regime should be achieved in these operating conditions. The steady-state analysis allows the numerical evaluation of the heat transfer coefficients between Cu and water; these are then used as input for the simplified, purely thermal (solid only) analysis of the actual transient problem for the full collector. The results of the simplified model, which allows a huge reduction of the computational effort, are successfully benchmarked against those of a comprehensive thermal–hydraulic simulation. The computed peak Cu temperature is below the acceptable limit under the steady-state (time averaged) heat load, but becomes unacceptably high if the actual transient heat load is considered, confirming the need for more effective sweeping strategies and/or optimized collector geometries with improved heat transfer capabilities.

The paper presents the experimental analysis and the characterization of an electrical capacitanc... more The paper presents the experimental analysis and the characterization of an electrical capacitance probe (ECP) that has been developed at the SIET Italian Company, for the measurement of two-phase flow parameters during the experimental simulation of nuclear accidents, as LOCA. The ECP is used to investigate a vertical air/water flow, characterized by void fraction higher than 95%, with mass flow rates ranging from 0.094 to 0.15 kg/s for air and from 0.002 to 0.021 kg/s for water, corresponding to an annular flow pattern. From the ECP signals, the electrode shape functions (i.e., the signals as a function of electrode distances) in single- and two-phase flows are obtained. The dependence of the signal on the void fraction is derived and the liquid film thickness and the phase’s velocity are evaluated by means of rather simple models. The experimental analysis allows one to characterize the ECP, showing the advantages and the drawbacks of this technique for the two-phase flow characterization at high void fraction.

Fusion Engineering and Design, 2015

Different configurations of water-cooled Cu collector for gyrotrons are investigated using the St... more Different configurations of water-cooled Cu collector for gyrotrons are investigated using the Star-CCM + CFD code, aimed at optimizing its thermal-hydraulic (TH) performance. Although the current collectors show a good performance, the collector can be subjected to transient heat loads, due to the spent electron beam, of up to several tens of MW/m 2 , and there is an interest to increase the gyrotron output power in the future. Furthermore, an optimized cooling will lead to improved reliability and lifetime of the collector. Starting from a hypervapotron (HV)-like collector, characterized by 100+ deep rectangular cavities with aspect ratio (AR) = 3, we present in the first part of the paper a single-cavity steady-state parametric analysis of the effect of AR on the heat exhaust capabilities. The investigation is then extended to other collector designs, including circumferential ribs and dimples, in order to assess the options for further improvements of the TH performance. The peak Cu temperature is computed by the code and its minimization is the target of the present optimization exercise. A self-consistent estimate of the heat transfer coefficient between collector and coolant is also obtained, which could be useful for fatigue and lifetime assessments. In the second part of the paper the most promising collector geometries identified in the first part are analyzed in the case of a transient heat load (vertical sweeping), first at the level of a single spatial period of the collector structure, then at the full-collector level. The results of the TH transient analysis are compared with both the results of the first part and with the transient purely thermal analysis of the full collector, showing for all geometries considered in this study a room for cooling efficiency improvement with respect to the HV-like design with AR = 3, at least in the operating conditions considered for this study (V ∼ 4 m/s, almost 100 • C sub-cooling).

Questo rapporto presenta l’attivita teorica e sperimentale svolta presso il Dipartimento Energia ... more Questo rapporto presenta l’attivita teorica e sperimentale svolta presso il Dipartimento Energia del Politecnico di Torino nello sviluppo della modellazione di uno spool piece (SP) per la stima dei parametri caratteristici di un deflusso bifase sulla base delle caratteristiche dello SP, della procedura di prova e dei risultati ottenuti nel corso delle campagne sperimentali Il dispositivo preso in considerazione e costituito da un tubo di Venturi e dalla sonda ad impedenza sviluppata dalla SIET per la stima della frazione di vuoto. I deflussi di interesse sono quelli ad elevata frazione di vuoto con flow patterns che coinvolgono il moto anulare, il moto anulare – disperso, il moto stratificato. Per integrare la matrice sperimentale ottenuta dal POLITO nella precedente annualita con la sonda in configurazione verticale, e stata condotta una campagna sperimentale con la stessa montata in configurazione orizzontale composta di 30 prove sperimentali in regime di “wet gas” (grado di vuoto...

The experimental analysis and the qualification of an Electrical Capacitance Probe (ECP), for the... more The experimental analysis and the qualification of an Electrical Capacitance Probe (ECP), for the void fraction measurement, is presented. The ECP, developed at the SIET, is used to investigate a vertical air/water flow, characterized by high void fraction. Mass flow rates have been analyzed between 0.094 and 0.15 kg/s for air and between 0.002 and 0.021 kg/s for water, corresponding to void fraction up to 90% and to annular and wavy-annular flow patterns. The ECP signals are used to obtain the geometrical shape functions (signals as a function of electrode distances) in single and two phase flows. The dependence of the signal by the void fraction is derived and other flow characteristics as the liquid film thickness and the phase velocities are evaluated by means of rather simple models. The experimental analysis allows to characterize the ECP, showing the advantages and the drawbacks of this technique for the two-phase flow characterization at high void fraction, and it provides a...

Questo rapporto presenta l’attivita teorica e sperimentale svolta presso il Dipartimento Energia ... more Questo rapporto presenta l’attivita teorica e sperimentale svolta presso il Dipartimento Energia del Politecnico di Torino per la caratterizzazione della strumentazione per la misura della portata in un deflusso bifase. Tale strumentazione, che consiste in uno ”Spool Piece” formato da un Venturi per la misura delle cadute di pressione ed una sonda ECP (Electrical Capacitance Probe) per la misura del grado di vuoto, e stata sviluppata per la misura della portate di fuga in simulatori integrati come lo SPES3. Studi condotti dal DEP nei PAR precedenti hanno mostrato la possibilita di utilizzare la misura delle cadute di pressione misurate dal Venturi per ricavare le portate delle due fasi una volta noto il modello di flusso bifase. Tale informazione puo essere ricavate dal segnale della sonda ECP ma occorre sviluppare una metodologia che permetta di interpretarlo correttamente. Nel corso del PAR2013 il DEP aveva testato lo “Spool Piece” in configurazione orizzontale, con un deflusso bi...

Nell’ambito dell’attivita LP2 “Reattori evolutivi INTD” del PAR2011, con riferimento ai reattori ... more Nell’ambito dell’attivita LP2 “Reattori evolutivi INTD” del PAR2011, con riferimento ai reattori di piccola e media taglia, il Dipartimento Energia del Politecnico di Torino, ha svolto una attivita di ricerca teorico-sperimentale con lo scopo di sviluppare e caratterizzare la strumentazione per la misura della portata in un deflusso bifase, da utilizzare nell’impianto SPES3. L’impianto SPES3 (Simulatore Pressurizzato per Esperienze di Sicurezza) e un simulatore integrato di reattori di piccola taglia modulari (SMR), capace di verificare la risposta del reattore a fronte di transitori incidentali e fornire dati sperimentali per la validazione di codici di calcolo per le analisi di sicurezza. Il circuito SPES-3, opera a elevate pressioni e temperature, e con elevate velocita dei fluidi. A causa delle condizioni di funzionamento, nella simulazione della rottura di una tubazione, si instaura un deflusso bifase nei condotti di collegamento tra il punto di rottura ed il componente di racc...

Nel presente documento e descritta la caratterizzazione di uno Spool Piece costituito da un ventu... more Nel presente documento e descritta la caratterizzazione di uno Spool Piece costituito da un venturi classico e da un Wire Mesh Sensor, installati in un condotto orizzontale interessato dal deflusso di una miscela aria-acqua. Questo Spool Piece e stato installato e testato in un circuito realizzato presso il laboratorio del Dipartimento Energia del Politecnico di Torino, che consente di realizzare un deflusso bifase aria-acqua in un ampio range di portate, permettendo lo studio di deflussi caratterizzati da differenti condizioni fluidodinamiche. Lo studio ha consentito di approfondire i lavori effettuati dal Politecnico di Torino nell'ambito dell'attivita LP2 "Reattori evolutivi lNTD" del PAR2008-09, in particolare nell'ambito della scelta degli strumenti da installare nelle linee di rottura di SPES3 con l'obiettivo di misurare la portata massica bifase delle due fasi che transita nelle linee durante i transitori incidentali. L'analisi dei segnali elettr...

Journal of Physics: Conference Series, 2015

The paper presents the results obtained with a spool piece (SP) made of a Venturi flow meter (VMF... more The paper presents the results obtained with a spool piece (SP) made of a Venturi flow meter (VMF) and an Electrical Capacitance Probe (ECP) in stratified two-phase flow. The objective is to determine the relationship between the test measurements and the physical characteristics of the flow such as superficial velocities, density and void fraction. The outputs of the ECP are electrical signals proportional to the void fraction between the electrodes; the parameters measured by the VFM are the total and the irreversible pressure losses of the twophase mixture. The fluids are air and demineralized water at ambient conditions. The flow rates are in the range of 0,065-0,099 kg/s for air and 0-0,039 kg/s (0-140 l/h) for water. The flow patterns recognized during the experiments are stratified, dispersed and annular flow. The presence of the VFM plays an important role on the alteration of the flow pattern due to wall flow detachment phenomena. The signals of differential pressure of the VFM in horizontal configuration are strongly dependent on the superficial velocities and on the flow pattern because of a lower symmetry of the flow with respect to the vertical configuration.

Transactions of the American Nuclear Society

In this study, the phase distribution of an air-water bubbly flow in a horizontal test section wi... more In this study, the phase distribution of an air-water bubbly flow in a horizontal test section with an inner diameter of 19.5 mm has been experimentally investigated by means of a Wire Mesh Sensor (WMS). The experimental test section is equipped with two quick closing valves (QCV) that allow the measurement of the volumetric void fraction in a length of 1300 mm. Two phase flow experiments have been performed at a constant temperature of 20 ± 2°C. In the present tests the superficial velocity ranges from 0.23 to 0.5 m/s for air and from 1.4 to 2.62 m/s for water.

Progress in Nuclear Energy, 2014

An experimental investigation of a vertical upward annular two-phase flow across a Venturi flow m... more An experimental investigation of a vertical upward annular two-phase flow across a Venturi flow meter has been performed for the measurement of two-phase flow parameters with reference to the experimental simulation of nuclear accidents, as LOCA, characterized by very high void fraction. The pressure drops between the inlet and throat section and between inlet and outlet (irreversible pressure losses) have been measured and analyzed. The Venturi flow meter is characterized by an inlet diameter of 80 mm and a throat diameter of 40 mm (b ¼ 0.5), with equal convergent and divergent angles (q ¼ 21). The instrument has been tested in a test section having an internal diameter (D i) of 80 mm and a total length (L) of about 4 m, with airewater two-phase flow at ambient pressure. The air superficial velocity ranged between 14 and 18 m/s while the water superficial velocity ranged between 0.0008 and 0.005 m/ s, so that the flow pattern was annular and the corresponding flow quality ranged between 0.78 and 0.96. The dependence of the pressure drops on the phases velocities has been analyzed and modeled as a function of the superficial velocities of the phases, and the mass flow rate of the two phases has been derived from the present test data when the annular flow pattern does not change in a significant way. The flow quality can be evaluated with an accuracy of 5% and the mass flow rate of air and water can be estimated with an accuracy better than 2% and 30% respectively.

Journal of Physics: Conference Series, 2014

The paper presents the research work performed at the Energy Department of the Politecnico di Tor... more The paper presents the research work performed at the Energy Department of the Politecnico di Torino, concerning the development of two-phase flow instrumentation and of models, based on the analysis of experimental data, that are able to interpret the measurement signals. The study has been performed with particular reference to the design of power plants, such as nuclear water reactors, where the two-phase flow thermal fluid dynamics must be accurately modeled and predicted. In two-phase flow typically a set of different measurement instruments (Spool Piece – SP) must be installed in order to evaluate the mass flow rate of the phases in a large range of flow conditions (flow patterns, pressures and temperatures); moreover, an interpretative model of the SP need to be developed and experimentally verified. The investigated meters are: Turbine, Venturi, Impedance Probes, Concave sensors, Wire mesh sensor, Electrical Capacitance Probe. Different instrument combinations have been tested, and the performance of each one has been analyzed.

Computational Methods in Multiphase Flow VII, 2013

A Wire Mesh Sensor (WMS), based on the measurement of the local instantaneous conductivity of the... more A Wire Mesh Sensor (WMS), based on the measurement of the local instantaneous conductivity of the two-phase mixture, has been adopted to characterize the air-water flow in a test section consisting of a horizontal Plexiglas (transparent) pipe of internal diameter 19.5 mm and total length of about 6 m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.145 to 31.94 m/s for air and from 0.019 to 2.62 m/s for water; the pressure ranges from atmospheric pressure to 3.7 bar. The observed flow patterns are stratified-bubble-slug/plug-annular. The void fraction profiles are derived from the sensor signals and their evolution in time and space is analyzed and discussed; the dependence of the signals from the measured fluid dynamic quantities is discussed too.

Journal of Physics: Conference Series, 2014

This works presents the characterization of a Spool Piece (SP), made up of a Classical Venturi an... more This works presents the characterization of a Spool Piece (SP), made up of a Classical Venturi and a Wire Mesh Sensor (WMS), that are installed in a horizontal test section, in which an air-water mixture flows. The test section consists of a horizontal Plexiglas pipe of internal diameter 19.5 mm and total length of about 7 m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.14 to 32 m/s for air and from 0.019 to 2.62 m/s for water; the pressure ranges from atmospheric pressure to 4 bar depending on the experimental conditions. The observed flow patterns are stratified-bubbly-slug/plug-annular. The instruments response is analyzed and discussed. From the signal analysis the mass flow rate of each phase is obtained. The developed model allows the evaluation of the mass flow rate with an accuracy higher than 20% in the 84% of the cases and with an accuracy higher than 10% in the 73.3% of the cases. Finally the introduction of the estimated parameters in the SP model is considered and discussed.

Journal of Physics: Conference Series, 2012

ABSTRACT A Wire Mesh Sensor, based on the measurement of the local instantaneous conductivity of ... more ABSTRACT A Wire Mesh Sensor, based on the measurement of the local instantaneous conductivity of the two-phase mixture, has been used to characterize the fluid dynamics of the gas-liquid interface in a horizontal pipe flow. Experiments with a pipe of a nominal diameter of 19.5 mm and total length of 6 m, have been performed with air/water mixtures, at ambient conditions. The flow quality ranges from 0.00016 to 0.22 and the superficial velocities range from 0.1 to 10.5 m/s for air and from 0.02 to 1.7 m/s for water; the flow pattern is stratified, slug/plug and annular. A sensor (WMS200) with an inner diameter of 19.5 mm and a measuring matrix of 16×16 points equally distributed over the cross-section has been chosen for the measurements. From the analysis of the Wire Mesh Sensor digital signals the average and the local void fraction are evaluated and the flow patterns are identified with reference to space, time and flow rate boundary conditions.

Experimental Thermal and Fluid Science, 2014

In the present work a Wire Mesh Sensor (WMS) has been adopted to characterize the air-water two-p... more In the present work a Wire Mesh Sensor (WMS) has been adopted to characterize the air-water two-phase flow in a test section consisting of a horizontal Plexiglas pipe of internal diameter 19.5 mm and total length of about 6 m. The flow quality ranges from 0 to 0.73 and the superficial velocity ranges from 0.145 to 31.94 m/s for air and from 0.019 to 2.62 m/s for water. The observed flow patterns are stratified-bubble-slug/plugannular. The WMS consists of two planes of parallel wire grids (16x16) that are placed across the channel at 1.5 mm and span over the measuring cross section. The wires of both planes cross under an angle of 90°, with a diameter D wire of 70 μm and a pitch equal to 1.3 mm. The void fraction profiles are derived from the sensor data and their evolution in time and space is analyzed and discussed. The dependence of the signals on the measured fluid dynamic quantities is discussed too. The main task is to predict which flow pattern will exist under any set of operating conditions as well as to predict the value of characteristic flow parameters.

Fusion Engineering and Design, 2015

Abstract The present gyrotron designs for EC plasma heating in nuclear fusion reactors require th... more Abstract The present gyrotron designs for EC plasma heating in nuclear fusion reactors require the safe exhaust of a power comparable to that injected into the plasma, in order to keep the maximum temperature below the acceptable value of 300 °C. In this paper, the commercial computational fluid dynamics (CFD) software STAR-CCM+® is used to analyze the thermal performance of the annular copper collector of a 1 MW ITER gyrotron, equipped with a hypervapotron structure made of annular fins with rectangular cavities of aspect ratio (depth/width) = 3, cooled by highly subcooled (90–100 °C) pressurized water flowing at ∼4 m/s. It is assumed that the simple vertical sweeping strategy is used to reduce the very high peak heat flux on the collector (up to 30 MW/m2 transient, 5 MW/m2 time average), due to the spent electron beam. The 2D steady-state conjugate heat transfer problem is solved assuming azimuthal symmetry and accounting for 2-phase flow. The single-cavity flow and heat transfer problem is considered first, to optimize the mesh and the selection of the turbulence model. For the operating conditions considered in this paper, the full collector (100+ cavities) solution shows that boiling occurs only in a limited number of cavities close to the peaks of the heat flux, with the vapor remaining trapped in the bottom of the cavities, i.e. no full hypervapotron regime should be achieved in these operating conditions. The steady-state analysis allows the numerical evaluation of the heat transfer coefficients between Cu and water; these are then used as input for the simplified, purely thermal (solid only) analysis of the actual transient problem for the full collector. The results of the simplified model, which allows a huge reduction of the computational effort, are successfully benchmarked against those of a comprehensive thermal–hydraulic simulation. The computed peak Cu temperature is below the acceptable limit under the steady-state (time averaged) heat load, but becomes unacceptably high if the actual transient heat load is considered, confirming the need for more effective sweeping strategies and/or optimized collector geometries with improved heat transfer capabilities.

The paper presents the experimental analysis and the characterization of an electrical capacitanc... more The paper presents the experimental analysis and the characterization of an electrical capacitance probe (ECP) that has been developed at the SIET Italian Company, for the measurement of two-phase flow parameters during the experimental simulation of nuclear accidents, as LOCA. The ECP is used to investigate a vertical air/water flow, characterized by void fraction higher than 95%, with mass flow rates ranging from 0.094 to 0.15 kg/s for air and from 0.002 to 0.021 kg/s for water, corresponding to an annular flow pattern. From the ECP signals, the electrode shape functions (i.e., the signals as a function of electrode distances) in single- and two-phase flows are obtained. The dependence of the signal on the void fraction is derived and the liquid film thickness and the phase’s velocity are evaluated by means of rather simple models. The experimental analysis allows one to characterize the ECP, showing the advantages and the drawbacks of this technique for the two-phase flow characterization at high void fraction.